The measurement of flame temperatures

To define the “temperature” of a flame, and to be sure that the concept has a unique meaning, is no easy task. Without entering into philosophic abstractions, it would seem that a working definition, which corresponds to the general idea of what is meant by “the temperature of a flame,” would be “the temperature of a solid body which is in thermal equilibrium with the flame.” Strictly, this is to argue in a circle, for "thermal equilibrium” means primarily “equality of temperature”; experimentally, it is satisfactory in that we may assume that thermal equilibrium corresponds to that thermal state which is eventually reached when the solid is left in contact with the flame for a sufficient time, provided the introduction of the solid does not alter the conditions.